Widely used heartburn and ulcer drugs such as Nexium, Pepcid and Prilosec can make people more susceptible to pneumonia, probably because they reduce germ-killing stomach acid, Dutch researchers found in a study of more than 300,000 patients.

The highest risks occurred with more powerful acid-fighting drugs called proton pump inhibitors, which are sold in the United States under such brand names as Nexium, Prevacid and Prilosec. Over nearly three years, users of these drugs faced almost double the risk of developing pneumonia compared with former users.

Users of another class of acid-fighting drugs that includes cimetidine and famotidine -- sold in the United States as Tagamet and Pepcid -- also faced an elevated risk.

The study was led by researcher Robert J.F. Laheij at University Medical Center St. Radboud in Nijmegen, Netherlands, and appears in Wednesday's Journal of the American Medical Association.

The acid in normal stomach fluids generally kills harmful bacteria; suppressing it with drugs to treat heartburn and ulcers may make the body more hospitable to such organisms, which may then infect the lungs and cause pneumonia, the researchers said.

These heavily promoted medicines are among the most widely prescribed drugs worldwide, with almost $13 billion in sales in 1998 alone, according to a JAMA editorial. Millions of Americans take these drugs, which are heavily advertised in "ask your doctor about. . ." TV commercials.